1. Field of the Invention
The invention relates to mobile communications comprising the exchange of video or data in association with a circuit switched call. Particularly, the invention relates to a method for performing a rich call capability indication in a mobile communication system.
2. Description of the Related Art
Rich Communication Suite (RCS) is a concept introduced by the GSM Association (GSMA) to represent advanced 3G services typically enabled by the IP Multimedia Subsystem (IMS) standards defined by 3G Partnership Project (3GPP). RCS contains typically session services that involve at least two media components transmitted between the session parties. The media components may comprise audio, video, still picture sequences, whiteboard, text, unstructured binary data and messages. Typically, there is a voice component that is augmented with a visual media component which may comprise still or moving image data. In order to cater for increased bitrates for the visual media component or any other media component transmitted in parallel with a voice component, the voice component may be transmitted over a Circuit Switched (CS) call, in other words, a bearer in accordance with the 3GPP standards 23.279 and 24.279 on combining CS services and IMS services. In this case the voice media component is transmitted in the form of a CS voice call, whereas a video media component is transmitted over Packet Switched (PS) data. The providing of the voice component as a CS call is useful, because GPRS PS data has a rather restricted upper bitrate capability, for example, in 80 kbit/s for downlink data when uplink bitrate is 20 kbit/s. The moving of the voice component to the CS side of a GSM system frees data transmission capacity from PS side. The quality of service obtainable via PS data for a voice call may also vary and may in some cases be inferior to the quality of service obtainable via a CS bearer. In 2G networks PS side is in a non-active state, which means that in networks not supporting Dual Transfer Mode (DTM), video sharing is not possible during a CS call.
Reference is now made to FIG. 1, which illustrates a communication system providing Combination of CS and IMS Services (CSI) in prior art. In FIG. 1 there are illustrated two Mobile Stations (MS), namely MS 100 and MS 180. MS 100 comprises an IMS client 102 and a CS Mobile Terminal (MT) function 104, whereas similar MS 180 also comprises an IMS client 182 and a CS Mobile Terminal (MT) function 184, respectively. MS 100 communicates with a Radio Access Network (RAN) 112 comprising a base station 112. MS 180 communicates with a RAN 170 comprising a base station 172. RAN 112 and RAN 170 may comprise, for example, a GSM BSS, a UMTS RAN, an Edge RAN, a 4G RAN or a short range RAN such as a WLAN or WiMAX LAN. Communicating with RAN 110 and RAN 170 there are respective PS Core networks 120 and 160, and CS Core networks 122, 162. PS Core networks 120 and 160 act as IP Connectivity Access Networks (IP-CAN) for IMS Core networks 130 and 150. IMS Core networks 130 and 150 communicate via an IP network 140 such as an inter-operator backbone network, a GPRS Roaming eXchange (GRX) network, Global Roaming eXchange (GRX) network, IP Packet eXchange (IPX) network, an Intranet or the Internet. CS Core networks 122 and 162 communicate via PSTN 142. Between IMS client 102 and IMS client 182 there is established a media component for video 190 via SIP signaling 191. SIP signaling 191 traverses in PS Core 120 an SGSN (not shown) and a GGSN (not shown). In IMS Core 130 SIP signaling 191 traverses a Proxy Call State Control Function (P-CSCF) and a Serving CSCF (S-CSCF), which are not shown in FIG. 1. SIP signaling 191 may also traverse a Inquiring CSCF (I-CSCF). Similar network elements may also be present in IMS Core 150. A CS signaling 193 between CS MT 104 and CS MT 184 may be in the form of Signaling System (SS) No. 7 ISDN User Part (ISUP) signaling, whereas the CS path 192 comprises a number of CS connections through CS Core 122, PSTN 142 and CS Core 162. It should be noted that the number of networks and the network structure in FIG. 1 is only for illustrative purposes, it is also possible, for example, that MS 100 and MS 180 share the same network operator and share the same PS and CS core networks. In this case there is no need to route signaling via IP network 140 or PSTN 142.
In order to support cases where a multimedia session is established to carry a visual media component such as video after the establishment of a CS to speech state, it is necessary to exchange information about the current capabilities of the serving RAN, the serving core network and the mobile equipment. The exchange involves information such as whether the terminal is capable of simultaneous CS and PS services, IP Multimedia (IM) status, UE capability version and personal Mobile Equipment (ME) identifier. IM status indicates whether the terminal supports IMS and willing to register to IMS in order to perform UE capability exchange via SIP OPTIONS operation as part of SIP signaling illustrated with arrow 191.
Reference is now made to FIG. 2, which illustrates capability information exchange between mobile equipment in a communication system in accordance with the 3GPP standards 23.279 and 24.279 in prior art. In FIG. 2 the starting point is that the user of MS 100 starts the establishment of a CS call towards MS 180. CS MT 104 sends a setup message 201 to an MSC (not shown) in CS Core 110. Setup message 201 comprises a User-User Information Element (UUE IE) that specifies protocol identifier for 3GPP capability exchange, radio environment set to value 1, IM status set to value “IM subsystem capable and willing to register to IM subsystem”, personal ME identifier and UE capability version. The UUE IE element in setup message 201 is transferred to an Initial Address Message (IAM) message 202, which is sent from CS Core 122 to PSTN 142. A further IAM message 203 carrying the UUE IE to an MSC (not shown) in CS Core 162 is sent from PSTN 142. The MSC in CS Core 162 sends the UUE IE to CS MT 184 within MS 180 in a setup message 204. CS MT 184 may send in response a progress message and an alerting message which are not shown for clarity purposes. When the called party answers, CS MT 184 within MS 180 sends a connect message 205 to the MSC in CS Core 162. Connect message 205 carries called party related capability information such as User-User Information Element (UUE IE) that specifies protocol identifier for 3GPP capability exchange, radio environment set to value 1, IM status set to value “IM subsystem capable and willing to register to IM subsystem”, personal ME identifier and UE capability version. The MSC in CS Core 162 sends an Answer Message (ANM) 206 comprising the UUE IE to PSTN 142 in response to the receiving of connect message 205. PSTN 142 sends an ANM message 207 comprising the UUE IE to the MSC in CS Core 122. The MSC in CS core 122 sends a connect message 208 comprising the UUE IE to CS MT 104 in response to ANM message 207.
After the CS call reaches the speech state via the exchange of messages 205-208, IMS clients 102 and 182 exchange their UE capabilities via SIP OPTIONS operations, provided that the IM status values they have receive are set to “IM subsystem capable and willing to register to IM subsystem” and that the radio environment is set to value 1, which indicates simultaneous support of PS and CS services or generally that IMS client 102 and IMS client 182 are capable of initiating a PS connection to IMS along with the ongoing CS call. Thereupon, IMS clients 102 and 182 register to their respective IMS cores 130 and 150 if no such registration has been performed. Thereupon, IMS client 102 sends a SIP OPTIONS operation 209 to IMS core 130. SIP OPTIONS operation 209 carries the UE capabilities of IMS client 102 within MS 100. The UE capabilities include information such as support for different possible IMS media types, particularly, video and still picture, media format parameters, personal ME identifier to identify which of the user's MEs the UE capability information is related to and UE capability version to label the capabilities carried in SIP OPTIONS operation 209. IMS core 130 sends the UE capabilities in a SIP OPTIONS operation 210 to IMS core 150. IMS core 150 sends the UE capabilities to IMS client 182 in a SIP OPTIONS operation 211. In response to SIP OPTIONS operation 211, IMS client 182 sends a 200 OK operation to IMS client 102, as illustrated with arrows 212-214. In order to inform its UE capabilities to the remote IMS client, IMS client 102 performs a similar SIP OPTIONS operation with IMS client 182, as illustrated with arrows 215-220. At some point in time the user of either MS 100 or MS 180 wants to initiate the establishment of a parallel video or still picture media component over IMS, which is illustrated with arrows 221-223. The establishment is performed using the SIP INVITE operation.
Significant amount of signaling would need to be performed between MS 100 and 180 in order to keep up-to-date information especially pertaining to radio environment capabilities, as the mobile stations may perform frequent handovers between 2G and 3G. The problem with existing solutions illustrated in FIGS. 1 and 2 is that the changed UE capabilities can not be indicated to the remote party using the SIP OPTIONS operations, if PS connection changes to non-active state, which means that it cannot be used when moved from 3G to 2G due to handover to a system that does not allow simultaneous CS and PS connection, for example, a 2G system. Dual mode transfer requires of the network support for simultaneous CS and PS data. Dual Transfer Mode (DTM) is defined, for example, in 3GPP specification 43.055.